1. Field of the Invention
The present invention provides a method and apparatus for measurement of velocity-type vector information related imaging and, more specifically, it provides such a system which preferably employs substantially tetrahedrally-oriented velocity-encoding gradients.
2. Description of the Prior Art
The advantageous use of non-invasive and nondestructive test procedures has long been known in medical and industrial applications. In respect of medical uses, it has been known that limiting a patient's exposure to potentially damaging x-ray radiation may be accomplished by the use of non-invasive imaging procedures such as, for example, ultrasound energy and magnetic resonance imaging. As to the latter, see generally, The Fundamentals of Magnetic Resonance Imaging by Hinshaw, et al., Technicare Corporation 1984.
In a general sense, magnetic resonance imaging involves providing bursts of radio frequency energy on a specimen that is positioned in a main magnetic field in order to induce responsive emission of magnetic radiation from the hydrogen nuclei or other nuclei. The emitted signal may be detected in such a manner as to provide information as to the intensity and phase of the response and the spatial origin of the nuclei emitting the responsive magnetic signal. The imaging is generally performed in a slice or plane or multiple planes, or three-dimensional volume. The information corresponding to the responsively emitted magnetic radiation is received by a computer which stores the information in the form of numbers corresponding to the intensity of the signal. The pixel value is established in the computer by a Fourier Transformation which converts the in-phase and out-of-phase signal amplitudes as a function of time to complex signal amplitude as a function of frequency. The signals may then be stored in a computer and may be delivered with or without enhancement to a video screen display such as a cathode-ray tube, for example, wherein the image created by the computer output will be presented through either black and white presentations varying in intensity or color presentations varying in hue intensity and saturation. See generally Conturo U.S. Pat. No. 4,766,381.
It has been known to determine velocity by ascertaining the phase shift in the magnetic resonance signal that is induced by motion along magnetic field gradients. If a specific motion-encoding gradient is applied, the resulting phase shift provides a measure of the velocity component along the direction of a specific gradient. It has been known to use a pair of acquisitions for each velocity component, one with and one without the motion-encoding gradient to thereby require a total of six sequential acquisitions, three of which are for the baseline phase. See, e.g. Pelc et al, Optimized Encoding for Phase Contrast Flow Measurement, Soc. of Magn. Reson. in Med., p. 475, Abstracts, (1990). In the 4-point null method one acquisition is without a velocity encoding gradient and the three remaining acquisitions have velocity-encoding gradients of equal magnitude which are sequentially positioned on a different one of the x, y and z axes. See, e.g., Conturo et al, Accurate Quantitative Imaging of Velocity Magnitude and Direction Using Phase-Nulled Orthogonal Bipolar Gradients, Abstract, SMRM, p. 25 (1987). Some of the problems with this approach are that velocity images are electronically noisy as the gradients do not range from a negative to a positive full scale, the image noise is dependent on velocity direction, and data is inefficiently acquired as the null phase adds noise without contributing velocity sensitivity.
Another approach that corrects for baseline phase shifts is the 6-point octahedral technique which corrects without a direct null phase acquisition. See, e.g., Dumoulin et al, Magn. Reson. in Med., Vol. 5, p. 47 (1987). It involves a balanced system because it has gradient vectors all of the same magnitude, whose vectors sum to zero and gradient pairs are in opposing directions along each of the three x, y, z axes. While the 4-point null method may be preferred because of a fewer number of acquisitions, it is still lacking the ideal signal-to-noise ratio.
It has been known to attempt to improve efficiency by providing a method which requires only four acquisitions, three of which have gradients bisecting pairs of x, y or z axes and one which is a direct baseline acquisition. See N. J. Pelc, et al., Optimizing Encoding for Phase Contrast Flow Measurement, Soc. of Magn. Reson. in Med., Book of Abstracts, p. 475 (1990). See also, M. Bernstein, et al., General Electric Technical Report #81, Radiology, 176 (August 1990). This system can be considered to be a hexagonal null system which has a null acquisition and three gradient vectors mutually separated by 60 degrees. The three gradient vectors are a subset of a full three dimensional array of vectors as occurs in hexagonal closest packing of spheres. This gradient vector configuration can, therefore, be deemed a 4-point hexagonal null system. This hexagonal null system is not balanced, as the center of the gradient mass is nonzero.
In spite of the foregoing, there remains a very real and substantial need for improving the relative slowness of operation and the undesirable low signal to noise ratio in magnetic resonance imaging of velocity motion and flow vectors.